The story spans 60 years, and is told in 6 segments by 6 different narrators. The main story is one of immortals who achieve their immortality by eating the soul of a psychic every quarter, opposed by a group of what are basically bodhisattvas, who after death regain their consciousness in a new body, normally near death such that its old inhabitant has vacated it. Plus the bodhisattva types have bodhisattva-like magical powers, yay!
[I always told my kids, if you do decide you want to be religious, go for Mahayana Buddhism and bodhisattva status. Bodhisattvas can fly, walk through fire, walk through walls, deflect arrows, be in multiple places at the same time, etc, - and choose your reincarnations! What's not to like?]
But this main story is left in the background for probably 2/3 of the book, while we follow different narratives in each of the segments. One character spans all 6 segments, but in some she plays a very minor role.
Meanwhile, the side stories are all engaging. Mitchell has a nice touch with meta-fu - like one of the narrators who is an author who swore never to write a book with an author as a protagonist, only to have his comeback novel have an author as a protagonist. (I so saw this author as being played in a movie by Bill Nighy, although he was 10-20 years younger than Nighy.) Another side character is a war correspondent, who very poignantly relates how the US invasion destroyed Iraq and the texture of the lives of its inhabitants.
Halfway through this book I rewatched "Cloud Atlas", based on one of Mitchell's earlier novels. It also has 6 main threads, loosely interconnected, with overtones of reincarnation. I had been meaning to do the rewatch - it is a complex movie - and I enjoyed it more the 2nd time, when I knew I needed to make more of an effort to keep the threads straight. Surprisingly - she generally doesn't do rewatches - my wife rewatched it with me and she enjoyed it too.
I'm tempted to read "Cloud Atlas" but it will probably be much later - too much else on the stack. I may try some of Mitchell's other novels tho. "The Bone Clocks" was definitely a worthwhile read.
Addendum: I went to mark this as read in my eBook reader and noticed I had put it in the fantasy shelf rather than sci-fi. A little of both, I guess.